1. Field of the Invention
The present invention relates to a knock sensing apparatus, and more specifically to a knock sensing apparatus for detecting knock vibrating of a multicylinder engine.
2. Description of the Prior Art
FIGS. 1A and 1B show a prior-art knock sensing apparatus disclosed in Japanese Published Unexamined Patent Appli. No. 60-4824, by way of example. In the drawings, a cylinder block 1 is formed with a cylinder 2, and a piston 3 is slidably fitted into the cylinder 2. On top of the cylinder block 1, a cylinder head 4 is so disposed as to form a combustion chamber 5 in cooperation with the cylinder block 1, the piston 3 and the cylinder head 4. The cylinder head 4 is formed with an intake passage 7 communicating with the combustion chamber 5 via an intake valve 6. An ignition plug 8 is attached to the cylinder head 4 to ignite a mixture within the combustion chamber 5 of a multicylinder engine, for instance.
Further, a knock sensor 9 is attached to the cylinder block 1 to detect knock vibration generated in each combustion chamber 5. The knock sensor 9 is connected to a knock detecting circuit 9. A including a bandpass filter 10, a comparator 11, a rectifier 12, a smoothing circuit 13 and an integrator 14. The knocking sensor 9 transduces engine knock vibration into an electric knock signal corresponding thereto; the bandpass filter 10 passes only the knock component signal of about 15 kHz; the rectifier 12 rectifies the bandpassed knock signal into a pulsating signal; and the smoothing circuit 13 smoothes the rectified pulsating signal to a dc signal. The output of the bandpass filter 10 is directly connected to a first input terminal of the comparator 11, but to a second input terminal of the comparator 11 via the rectifier and the smoothing circuit 13. Therefore, where no knock vibration is detected by the knock sensor 9, the comparator 11 outputs no output signal, because the signal levels of the two input terminals of the comparator 11 are almost the same; however, where knocking vibration is detected, the comparator 11 outputs a knock signal of a predetermined voltage level, because there exists a difference in signal level between the first and second input terminals of the comparator 11. The knock signal is integrated by the integrator 14, so that a knock signal whose voltage level varies according to the frequency of knock vibration is applied to an ignition circuit 15. The ignition circuit 15 controls ignition timing of the ignition plug 8 in accordance with the voltage level of the knock signal obtained through the integrator 14 in such a way that ignition timing is delayed whenever knock vibration is generated.
The structure of the knock sensor 9 is disclosed in detail, for instance, in Japanese Published Unexamined Utility Model Appli. No. 1-134229, as shown in FIG. 1(C). In the drawing, a casing 16a of the knock sensor 9 is formed with a tapered threaded portion 16a so as to be screwed into a threaded hole of the cylinder block 1. On the other hand, a pair of piezoelectric elements 17 are housed within the casing 16 and fixed to the casing 16 via a weight member 18 with a bolt 19. Since the piezoelectric elements 17 can transduce mechanical energy (knock vibration) to electric energy (knock signal), the transduced knock signal can be obtained through a terminal 120 connected to the piezoelectric elements 17.
In the prior-art knock sensing apparatus as described above, however, since the cylinder block through which knock vibration is transmitted and detected is made of cast iron or aluminium alloy and formed into a complicated shape in order to reduce engine vibration noise (because cast iron or aluminium alloy is low in vibration transmissibility), there exists a problem in that knock vibration generated in each cylinder combustion chamber 5 is reduced during propagation through the cylinder block 1 and therefore knock vibration is not effectively transmitted to the knock sensor 9, so that it is impossible to reliably detect the presence of knock vibration of a cylinder remote from the knock sensor, in particular in the case of a multicylinder engine.